When listening in the speech mode a subject can identify the phonetic class of a sound but cannot discriminate in any way between acoustically different sounds that have been classified as the same. This phenomenon is called categorical perception. The general objective of this research project is a detailed examination of the relationship between absolute identification and differential discrimination of several classes of speech sounds. We propose to examine the experimental conditions under which differential discrimination of speech sounds is limited by absolute identification--the idealized case of categorical perception. Recent findings have shown that categorical perception is primarily the result of the differential availability of auditory short-term memory for the acoustic cues that distinguish different classes of speech sounds rather than the differences which may underlie the production of these sounds. The present research represents a detailed elaboration of these findings on the nature of categorical perception. In conducting this research we hope to be able to specify quantitatively the degree to which categorical perception is dependent on the specific auditory and phonetic properties of speech stimuli (i.e., consonant vs. vowel) and the degree to which categorical perception is dependent upon the specific experimental conditions employed in previous speech perception experiments. Studies are planned which will employ techniques and methods from acoustic phonetics and speech synthesis with the intent of establishing quantitatively some of the "special" characteristics of perception in the speech mode.